Why is Pluto not a planet?

In 1930, Clyde W Tombaugh completed the search for Planet X, the outermost planet at the far reaches of the solar system. This object was named Pluto. Then, on 24 August 2006, the IAU (International Astronomy Union) formally voted on a new definition of the word 'Planet', which excluded Pluto. For seventy six years, our solar system had been understood to have nine planets, then suddenly it went back to eight. The response was furious.

Both the astronomical community and the general public were up in arms over this seemingly arbitrary decision. To the public, it was a travesty that school textbooks would have to be re-written, and astronomers felt that the vote had been manipulated by scheduling it for the final day of the General Assembly in Prague when most of the attending astronomers had already flown home. The state of Illinois even passed a law decreeing that Pluto was still a planet (rather like the attempt to legislate a new value for Pi!), and to this day there's any number of websites, clubs and T-shirt vendors devoted to reversing the decision. So what was going on? What did this international body of astronomers have against poor Pluto?

The Problem

Once Pluto had been discovered, named and classified, our picture of the solar system was pretty clear-cut. One star, nine planets, a bunch of comets and a multitude of asteroids broadly spread out in a region of space roughly between Mars and Jupiter. But there was one huge problem that had managed to slip through the cracks: there was, and never had been, a definition for the word "Planet". Everybody just knew what a planet was, which actually meant that nobody knew what it was. It was just accepted that planets were one of nine large things orbiting the sun, which tells you almost nothing about planets at all. While this was a hugely unscientific way to approach things, in practice it didn't matter since there were only nine known planets in the entire universe. Nobody using the word risked confusing their audience.

Shortly after the discovery of Pluto, astronomers started speculating about what might lie further out. Comets were well known and studied, but every now and then a very bright new comet would appear, following an orbit so large that it must have fallen in from way beyond the orbit of Neptune. Such comets would shoot by dramatically, and then vanish once more into the cold depths of space never to be seen again. Astronomers began speculating that there might be a a vast number of tiny icy objects orbiting out beyond Neptune, and that occasionally a gravitational interaction would eject one inwards, where it would fall past the sun and become one of these Long Period Comets. In 1950, one Gerhard Kuiper speculated that these objects could have been arranged in a structure similar to the asteroid belt, only colder and further away. Despite the fact that Kuiper didn't believe that such a belt actually existed (It was still thought that Pluto must be about the size of Earth, and so would have scattered such a belt a long time ago), it now bears his name: The Kuiper Belt.

In 1978, when Pluto's largest moon was discovered, and astronomers were able to determine just how small Pluto was, the idea of the Kuiper Belt became plausible, and the first object in the belt was discovered in 1992 by David Jewett and Jane Luu. This opened a floodgate of new Trans-Neptunian Objects (or TNO, meaning literally: "Things that are past Neptune". Scientists like using big words to describe simple things!), of which there are now many hundreds known and catalogued. This put Pluto in an uncomfortable situation of having a lot more in common with the TNO's than with any of the other 8 planets. After all, it was extremely small (two third's the size of our own moon, and not much larger than the largest asteroid, Ceres), its orbit was so elliptical that sometimes it was actually closer to the Sun than Neptune, and it's orbit was inclined away from the ecliptic.

Then, in 2003, a team of astronomers at the Palomar Observatory discovered a TNO with a diameter of 2500 km - larger than Pluto.

As if all that wasn't bad enough, planets were starting to be found orbiting other stars. The technology couldn't detect anything on the scale of our own familiar planets, but it was finding monstrous super-planets many times the size of Jupiter. These "planets" were so huge that they were on the verge of igniting and becoming stars - they were the often-theorised Brown Dwarfs. These things seemed to sit on the boundary between star and planet, highlighting more and more the need for a strict definition.

The Solution

Astronomers began debating how to resolve the mess. Unfortunately it was harder than it seemed to come up with a scientifically rigorous definition that wouldn't change the common meaning of the word. Some suggestions would eliminate Pluto, others would end up giving us over three hundred planets around our own Sun (as the number of discovered TNO's and exoplanets increased), and others lacked clarity. After much debate at the 2006 General Assembly of the IAU in Prague, a vote was held during the closing ceremony, and the issue was considered closed. The new definition was as follows:

A Planet is defined as any celestial body that:

Is in orbit around the Sun

Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (meaning that it is round, because it's big and heavy enough that it squeezes itself into an almost round shape)

Has cleared the neighbourhood around it's orbit.

It's not hard to see why astronomers revolted. It was highly irregular to relegate the decision on such an important topic to a quick vote during the closing ceremony, and many have suggested that this was done deliberately to prevent dissenting astronomers from voting against the new definition. The new definition seems to raise more questions than it answers, as well, in that it seems to preclude planets around stars that aren't the Sun. The third term especially is a problem, since to make it fit many of the existing planets you have to do a lot of hand-waving to explain what is meant by "Cleared the neighbourhood". It's not at all clear, and it's plain why so many astronomers claim after the fact that had they been present, they would have voted against it.

Nevertheless, for all its problems, the new definition is a step forward. It is a vast improvement on the old situation, and the general public's issue with it (the "demotion" of Pluto) is a purely emotional one. It is widely accepted within the astronomical community that had we known more about the solar system in 1930, Pluto would never have been classified as a planet in the first place. And anyway, what's in a name? Pluto continues on its weird orbit, out in the cold wastes of the outer solar system, blissfully unaware and uncaring of how we choose to categorise it.